Mac OS X Password Recovery
Because OS X just logs you without a password by default (like Windows 95 used to), you can really forget your password as I did with a now ancient iMac (the one that looked like a lamp). So how do you recover. Well, Google is my friend and here is what Elliot says:
Although there’s no real way to recover a lost root password, you can change the root password even if you do not know the current one. You must have physical access to the machine in order to accomplish this task.
1. Click Restart at the login window
2. While the computer is restarting, hold down “Command-S” until you see text scrolling through the window. This boots the computer into single user mode.
3. At the Localhost% prompt type:
/sbin/mount -uw /
You will then see various services starting up.
4. When the Localhost% prompt reappears, type: passwd [username] It will then ask you to type the new root password twice, so do so. (Though I’ve not tried it, it appears you can type niutil -list . /users to get a list if users if you do not know the username….if someone wants to verify this, I would be most appreciative).
5. After entering the new password, type: reboot
6. At the login window, enter the username with the new password. Once you are logged in, you can use the Multiple Users application (/Applications/Utilities) to change your user’s password, or create a new user account.
Note: this may not work for newer versions of OS X; I think they did something about it in a security update. I hope this helps you with Macintosh password recovery.
http://www.ofzenandcomputing.com/zanswers/43 has another way.
You’ve forgotten the passwords to all of the user accounts on your system. You don’t even remember the Administrator password. End of the line, right? Wrong. If you have the Mac OS X CD that was used to install your system in the beginning, then you can use that CD to gain access to your Mac and change the password for any or all of the existing accounts. For detailed instructions on how to do this, visit the Apple support document titled “Changing your Administrator password“. After you’ve finished resetting your password or passwords, be sure to store away the system CD in a safe place since anyone who has it esentially has unrestricted access to your Mac.